The use of composite dielectrics, in particular the composite of paper and polymer film, to extend the life of electrostatic capacitors under high voltage stress is known. For example, U.S. Pat. No. 3,048,750 of Netherwood et al. discloses the use of a composite dielectric having one layer of dielectric with a volume resistivity of at least five times the volume resistivity of the dielectric of the other layer. In particular, one layer of nonporous polymer film having a high resistivity and one layer of porous paper having a low resistivity is filled with impregnates of low resistivity. The invention is to place an anode between layers of lower resistivity dielectric (layer-LR) and to place a cathode between layers of higher resistivity dielectric (layer-HR). This means that for a given voltage across a composite dielectric, 83% of the voltage should be across the layer-HR and 17% across the layer-LR. Since it is generally accepted in the art that degradation of an electrostatic capacitor takes place at the anode, this invention provides means to lower the electrical stress at the anode and thus extend the life of capacitors.
Yagitani in U.S. Pat. No. 3,857,073 further improves the overvoltage endurance over Netherwood et al. as above by having the same composite dielectric arrangement as that of Netherwood et al., except that one of a pair of electrode foils is narrower in width, with its both edges made recessed foil edges in relation to the other electrode and both of its surfaces kept in contact with paper (layer-LR).
Further improvement over Yagitani is made by Mercier et al. in U.S. Pat. No. 4,323,948 by having the narrower electrodes rounded and smooth edges such as by folding. Mercier et al. further associate the layer-LR to that of high dielectric constant and the layer-HR to that of low dielectric constant.
Each of U.S. Pat. Nos. 3,857,073, and 4,323,948 are directed to capacitors containing the composite dielectrics and the narrower electrodes in contact with the layer-LR. Particularly, the composite dielectric includes a layer of porous paper filled with impregnant and a layer of non-porous polymer film.